The cord-grass marshes of south Louisiana are nurseries for baby shrimp, stalking grounds for blue crabs, and barriers that slow down waves before they bite off more of the mainland. On Friday, they were becoming defenseless sponges for sticky, dark oil.

The locals said the foul-smelling mass had the goopy look of chocolate mousse. The scientists said the enormous slick had the potential to bring environmental ruin to this treasured coastline.

The oil is spilling out of the seafloor at 5,000 barrels a day -- the equivalent of 210,000 gallons -- maybe much more, from a well about 50 miles out in the Gulf of Mexico, and it could soon eclipse the volume of the infamous Exxon Valdez spill in 1989. That disaster spilled oil onto rocky Alaskan beaches, but it is at least possible to wash oil off a rock. In the Gulf, the oil is floating into wetlands that could hold on to its toxins for years.

And, scientists said, the spill's damage could be magnified by its awful timing.

Among the animals that live along the Gulf Coast, this is the time for hatching and rearing: Species as diverse as pelicans, shrimp and alligators are all reproducing, or preparing to. That could bring sensitive young animals in contact with toxic oil or cause their parents to plunge into oily waters looking for food.

Already, rescuers from Delaware were using Dawn blue dish soap to clean the first bird found to be "oiled" in the disaster, the Associated Press reported. The bird, a young northern gannet, had been turned from white to black.

"I can't imagine we're not going to have some mass casualties" among these birds, said Michael Parr of the American Bird Conservancy. "It's got to be about the worst time right now" for an oil spill to hit.

One environmentalist said the scenario created "a potential mega-disaster." Along the Gulf, they were waiting for it with a dread usually devoted to hurricanes.

"The magnitude and the potential for ecological damage is probably more great than anything we've ever seen in the Gulf of Mexico," said Nancy Rabalais, a scientist who heads the Louisiana Universities Marine Consortium, a research center in Cocodrie, La. "Once it hits the shoreline, it'll get into everything."

The oil has been flowing since April 22, when a drilling rig called the Deepwater Horizon blew up and sank 50 miles out in the Gulf.

By late Friday, federal officials estimated that the slick had reached land in Louisiana's isolated southeast corner and was being pushed ashore by strong winds.

It had already surrounded some of the Chandeleur Islands off Louisiana, where thousands of brown pelicans are roosting. The U.S. Fish and Wildlife Service, which oversees the islands, has protected them with a ring of floating booms.

That barrier so far has stopped the oil, despite heavy seas.

"We hope it holds," said Tom MacKenzie, an agency spokesman.

In south Louisiana's Plaquemines Parish, fishermen were already seeing the oil as a thick, brown sludge, washing toward the town of Venice. "It's the chocolate mousse, that's the term they've been using," said Albert "Rusty" Gaudé, a state extension agent who works with fishermen there. He said it had left many fearful that crabs, oysters and shrimp -- part of a Louisiana industry that produces 10 percent of the country's seafood -- could be devastated.

"There's only two things that are happening in Venice, and that's seafood and oil," Gaudé said. "Could be catastrophic for both of them."

In the rest of the region, the oil was still unseen -- scientists and fishermen were left to wait and worry.

"There's a lot at risk here," said Jane Lubchenco, who heads the National Oceanic and Atmospheric Administration. She noted that 40 percent of the coastal wetlands in the continental United States are in Louisiana. "Ninety-seven percent of commercial fish and shellfish in the Gulf depend on estuaries and wetlands during their life cycle," she said.

In the Gulf, some scientists worried about the marsh itself: In south Louisiana, the oil was hitting wetlands dominated by Spartina grass, with huge clumps of dead grass underwater. Thomas Shirley, a professor at Texas A&M University's Harte Research Institute for Gulf of Mexico Studies, said that posed a problem that Exxon Valdez did not.

"There's no way to wash the oil out of a Spartina marsh," Shirley said. Instead, he said, it could take years to leach out with the tides. "It's just a big sponge."

That could mean trouble for the marsh itself, and for the land behind it. If marshes die, that could remove a key natural barrier that takes the punch out of waves before they hit Louisiana's fast-eroding coast.

At Louisiana State University, Professor Ed Overton said his worries grew after a university laboratory analyzed a sample of the oil. "It's not terribly toxic, but it does appear to be terribly sticky," he said, which meant it could coat blades of grass and kill them.

Oil is damaging for birds because it coats their feathers, destroying the natural chemistry that keeps them buoyant, warm and able to fly. When birds "preen" and try to remove the oil, they can swallow it and be poisoned.

So it is terrible timing that, at this time of year, huge numbers of birds converge on the marshes and empty barrier islands on this stretch of the Gulf Coast. Some are just stopping to recharge after a long flight over the Gulf from South America. But others have come to stay, preparing to raise their young in nests in the marsh and along sandy beaches.

Among other wildlife, scientists said, some fish would swim away from the oil -- provided they can flee to an oil-free place. The situation is worse for dolphins, which must surface twice a minute to breathe: They would inhale fumes from the oil.

The natural rhythms of many other species make them also particularly vulnerable. Tiny shrimp, just hatched and only a few millimeters across, will be swimming at the oily surface. Alligators, nearing their nesting season, might ingest the oil as they feed. Sea turtles will be coming ashore to lay their eggs on beaches.

Scientists said that huge die-offs are not assured: The wind could shift and blow the slick offshore, or the spilling oil might finally be capped, mitigating the damage.

But as of Friday night, neither of those fervently hoped-for things had come to pass.

"I'm going to be honest with you, it's got a lot of people very fearful," said Avery Bates of the Organized Seafood Association of Alabama in Bayou la Batre. He said the slick was expected to hit their stretch of coastline Sunday.